Carbon nanotubes are very small tube-shaped structures having the composition of a graphite sheet, formed as a tube. Carbon nanotubes produced by arc discharge between graphite rods were first reported in an article by Sumio Iijima entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58).
Carbon nanotubes typically have excellent performance; in particular, excellent electron emission capability. Furthermore, carbon nanotubes can carry high electric currents reliably. Due to these properties, carbon nanotubes are considered to be an ideal field emission material for application in a variety of field emission devices, Such as vacuum electron sources, field emission flat panel displays, etc.
A carbon nanotube-based field emission device typically includes a cathode base and a carbon nanotube layer acting as field emitter formed on the cathode base. Recently, both the direct growth method, such as chemical vapor deposition (CVD), and the screen-printing method using carbon nanotube-based paste have been recognized as two promising techniques for the formation of the carbon nanotube layer on the cathode base.
Specifically, the direct growth method for the formation of a conventional carbon nanotube-based field emission device usually includes the steps of: providing a cathode base with a catalyst layer formed thereon; introducing a carbon source gas flowing over the catalyst layer; forming a carbon nanotube layer originating from the catalyst layer on the cathode base by way of a chemical vapor deposition process. The carbon nanotube layer is composed of a plurality of carbon nanotubes with top ends acting as field emitter tips. The top ends locate away from the cathode base. However, because distribution of the top ends is tangled, an emission current of the field emission device is non-uniform, as shown in FIG. 8.
The screen-printing method for the formation of a conventional carbon nanotube-based field emission device usually includes the steps of: providing a carbon nanotube-based silver paste which includes a plurality of carbon nanotubes randomly and multi-directionally disposed therein; screen-printing the paste on a cathode base; solidifying the paste formed on the cathode base. However, because the carbon nanotubes disposed in the paste are random and multidirectional, an emission current of the field emission device is not uniform too, as shown in FIG. 9.
What is needed is to provide a method for fabricating a carbon nanotube-based field emission device which can achieve a relatively more uniform emission current.